Dramatic Morphology Control in the Fabrication of Porous Polymer Films**

Transcription

1 DOI: /adfm Dramatic Morphology Control in the Fabrication of Porous Polymer Films** By Luke A. Connal, Robert Vestberg, Craig J. Hawker, and Greg G. Qiao* Highly ordered, porous honeycomb films are prepared by the breath-figure (BF) technique using dendron-functionalized star polymers as precursors. By changing the nature of the dendritic end groups, dramatically different porous morphologies can be produced. Three series of star polymers are prepared with both the size of the 2,2-bis(methoxy)propionic acid (bis-mpa)-based dendron end group and the dendron functionality being varied. Star polymers end-functionalized with acetonide-protected dendrons (generations 1 to 4) are initially prepared and the acetonide groups subsequently deprotected to yield hydroxylfunctionalized star polymers. Modification of these hydroxyl groups with pentadecafluorooctanoyl chloride yields a third series of functionalized star polymers. The resulting star polymers have surface groups with very different polarity and by utilizing these star polymers to form honeycomb films by the BF technique, the morphology produced is dramatically different. The star polymers with amphiphilic character afford interconnected porous morphologies with multiple layers of pores. The star polymers with pentadecafluorooctanoyl end groups show highly ordered monolayers of pores with extremely thin walls and represent a new porous morphology that has previously not been reported. The ability to prepare libraries of different dendronized star polymers has given further insights into the BF technique and allows the final porous morphology to be controllably tuned utilizing the functional chain ends and generation number of the dendronized star polymers. 1. Introduction Functional porous films with specific surface chemistries are leading to many new devices in applications ranging from catalysis, [1,2] microreactors, [3] photonics, [4,5] chromatography, [6] sensors, [7,8] and biosensors. [9,10] One cost-efficient and versatile method for producing ordered porous films is via the breath-figure (BF) technique. Breath figures are arrays of liquid drops formed during vapor condensation onto a surface. The investigation of such a phenomenon dates back to work by Aitken [11] and Rayleigh [12] as early as 1911 and more recently by Beysons and co-workers. [13] In 1994, the group of Francois [*] Prof. Greg G. Qiao, Dr. Luke A. Connal Polymer Science Group Department of Chemical and Biomolecular Engineering The University of Melbourne Parkville, Victoria 3010 (Australia) gregghq@unimelb.edu.au Dr. Robert Vestberg, Prof. Craig J. Hawker Materials Research Laboratory and Departments of Chemistry, Biochemistry, and Materials University of California, Santa Barbara Santa Barbara, CA (USA) [**] Financial support from the NSF MRSEC Program DMR (MRL-UCSB), National Institutes of Health as a Program of Excellence in Nanotechnology (HL080729), and an Australian Research Council s Discovery Grant (DP ) is gratefully acknowledged. The authors would also like to thank the Royal Australian Chemical Institute (Polymer Division) for travel support through the O Donnell prize (LAC). reported a method that utilizes BFs to form self-assembled, highly-ordered honeycomb films, by casting a polymer solution from a volatile solvent under a flow of humid air. [14] In this process, water droplets condense as micrometer-sized spheres (breath figures) from the humid environment onto the polymer solution surface, initiated by the rapid evaporation of the solvent. These water droplets self-assemble into an ordered hexagonal pitch, induced, in part, by convection currents. The polymer phase then precipitates around these ordered templates stabilizing them from coalescence. The process effectively immobilizes the water templates and, upon further evaporation, leaves an ordered three-dimensional honeycomb structure. This technique has many advantages over other processes: it is simple, cheap, and the structure is easily controlled by changing the casting conditions. [15] Importantly, no extra steps are needed to remove the water template as the water simply evaporates from the film. These films have been produced from a range of polymeric materials, for example: rod-coil block copolymers, [14] block copolymers, [16] amphiphilic copolymers, [17] star polymers, [18 20] and core crosslinked star (CCS) polymers. [21 24] All of these polymers are capable of forming spherical structures in solution, which has been reported to be a critical factor in this casting technique. [14] Another important development in the area has been the coating of nonplanar surfaces, utilizing star polymers with low glass-transition temperatures. [22 25] Because of the inherent mechanism utilizing water as a template the effect of polymer functionality (i.e., hydrophobicity) becomes an 3706 ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2008, 18,

2 important characteristic, playing an important role in the formation of such films and the final porous morphology. While studies have shown that polar groups line the pore walls, [26,27] to the best of our knowledge there has been no study demonstrating that the final honeycomb morphology can be systematically manipulated by controlling the polymer functionality. To fully investigate this feature, chain-end-functionalized dendronized star polymers were chosen as the test vehicle because of the large number of chain ends, the number and nature of which can be accurately controlled. Recently, we developed a strategy to synthesize CCS polymers functionalized at the end of each arm, with a dendron up to generation 5. [28] The power of dendritic macromolecules is evident by the ability to prepare star polymers with hydroxyl groups at the end of each arm, with the number of hydroxyl groups being controlled by the generation number. By synthetically modifying these hydroxyl groups, this synthetic strategy gives an ideal platform for the systematic variation of functional group number and nature. Herein, we prepare star polymers with polystyrene (PS) arms end-functionalized with dendrons (G1 to G4) based on 2,2-bis(methoxy)propionic acid (bis-mpa). [29 33] From this base series of star polymers two additional series of functional stars, with a controlled number of surface groups and different hydrophobic character, have been developed. The original acetonide-functionalized star polymers have a hydrophobic character that, after deprotection, gives hydrophilic, hydroxylfunctionalized stars. These hydroxyl groups can be further reacted with pentadecafluorooctanoyl chloride, creating a series of stars with very hydrophobic surface groups. This strategy enabled the synthesis of three series of star polymers with varying hydrophobicity, by controlling both the surface functional group and the generation number of the dendron. In this Full Paper, we utilize these star polymers to demonstrate, for the first time, the dramatic effects that functionality has on the final morphology of honeycomb films prepared by the BF technique. 2. Results and Discussion The arm first strategy was utilized to prepare dendronfunctionalized star polymers, protected at the periphery by acetonide groups. Initially, dendron-functionalized initiators (G1 G4) were used to initiate the polymerization of styrene. These dendron-functionalized polystyrenes were then reacted with divinyl benzene to synthesize CCS polymers (SP1 4), as shown in Scheme 1. Figure 1 shows the gel permeation chromatography (GPC) traces, and Table 1 summarizes the characterization data for the star polymers synthesized Functional Group Transformations Scheme 1. Preparation of an acetonide-protected G4-functionalized star polymer: 1) Polymerization of styrene; bulk, 110 8C, 2 h; Molar ratio styrene/n,n,n 0,N 00,N 00 -pentamethyldiethylenetriamine (PMDETA)/copper(I) bromide/acetonide-[g4]-co 2 (CH 2 ) 2 CO 2 C(CH 3 ) 2 Br 100:2:1:1. 2) Polymerization of divinyl benzene (DVB); anisole, 100 8C, 40 h; DVB/PMDETA/CuBr/macroinitiator 15:2:1:1. The acetonide rings of the star polymers (SP1 4) were deprotected using Dowex acidic resin in methanol/tetrahydrofuran (THF) mixtures at 508C (Scheme 2) to give star polymers that have hydroxyl functionality at the end of each arm (SP5 8). The use of well-defined dendrimers allows a range of star polymers with a varying number of hydroxyl groups per star to be prepared. For example, the G4 star (SP8) has 16 OH groups per arm and approximately 190 OH groups per star. With the appropriate functionalization strategy, the hydroxyl functionality can then be transformed into other functional groups. In this instance, in order to create a very hydrophobic end group, the hydroxyl groups of stars (SP5 8) were reacted with pentadecafluorooctanoyl chloride (Scheme 2), creating unique star structures with varying amounts of fluorine localized at the periphery of the star polymer. This strategy allows a library of star polymers with a systematic variation in hydrophobic character to be prepared: hydroxyl function- Adv. Funct. Mater. 2008, 18, ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

3 ality (hydrophilic), acetonide functionality (hydrophobic), and the perfluoroalkyl functionality (very hydrophobic) Preparation of Honeycomb Films Figure 1. Crude gel permeation chromatographs, displaying the normalized differential refractive index (DRI) from a series of acetonide-functionalized star polymers. The high molecular weight peak (at approximately 22 ml) is the star polymer, and the lower molecular weight species at higher elution volumes is unconverted linear macroinitiators or possibly smaller star polymers with low arm numbers. Fractional precipitation afforded low polydispersity star polymers. Table 1. Summary of acetonide-protected dendron-functionalized star polymers[a]. Entry Macroinitiators Core Crosslinked Star Polymers DP n [b] End group[c] M W [b] PDI[b] No. arms[d] Arm conv.[e] SP1 85 Act-G T SP2 90 (Act) 2 -G T SP3 100 (Act) 4 -G T SP4 110 (Act) 8 -G T [a] Reaction conditions: [macroinitiator] 0 ¼ [CuBr] 0 ¼ [PMDETA] 0 /2 ¼ [DVB] 0 /15, anisole (15 mm, with respect to the macroinitiator concentration), 100 -C, 40 h. [b] Degree of polymerization (DP n ) and weight-average molecular weight (M w ) were measured by gel permeation chromatography equipped with multiangle laser light scattering (GPC-MALLS). PDI: Polydispersity index. [c] End group of the macroinitiator. [d] Number of arms calculated from the formula: f w,arms M w, star /M w, arms. [e] Calculated based on the integration from GPC concentration detector (DRI), arm conv. ¼ f w,arms A 1 /(A 1 RA 2 ), where A 1 is the area corresponding to the star polymer, A 2 is the area corresponding to the remaining linear polymer, and f w,arms is the weight fraction of arms in the CCS polymer. This library of star polymers, with very different surface chemistries, was then used to prepare honeycomb films by the BF technique. In previous studies we have found residual linear components in the star mixtures have little effect on the overall morphology. In fact, films with some linear components displayed improved properties, showing less brittle films. [21] We also verified this for this system by isolating pure star polymer in the SP3 mixture by fractional precipitation. Honeycomb films formed from pure star polymer (SP3) were identical to the star linear mixtures. However, pure lineardendron block copolymer solutions did not form ordered honeycomb structures. Our earlier studies [21 22] and other reports [19] have shown that small amounts of linear polymers do not affect the formed porous structures, in fact, they enhance the mechanical strength of the film. Therefore, in this study the star mixtures were used as synthesized. The synthesis strategy utilized enables two systematic studies to be carried out: firstly, the effect of the generation number (i.e., amount of functional group), and secondly the effect of the functional group (i.e., the hydrophobicity of the end group). When honeycomb films were cast from acetonide-functionalized stars (SP1 4), a droplet (20 ml) of a star polymer/ benzene solution (10 mg ml 1 ) was cast onto a glass cover slip under a flow of humid air (3 L min 1, 70% R.H., relative humidity). Figure 2 shows the resulting structures from G1 to G4-functionalized CCS polymers (SP1 4). All star polymer mixtures were able to form honeycomb-like porous films, with generation 1 to 3 producing ordered honeycomb films. It is worth noting that the porous morphology has two distinct features: an open pore diameter and an underneath cell diameter. The open diameter is seen in Figure 2b, d, and f, where the black areas of the film show the open pore size. Around these dark areas is a lighter region in a hexagonal pattern, and this structure is the underneath structure, indicating the pore has a smaller opening at the surface. Acetonide-G4 star polymers (SP4, Fig. 2g and h) show disordered honeycomb morphology often with completely closed pores on the surface Honeycomb Materials from Hydroxyl-Functionalized Star Polymers Honeycomb films were cast from hydroxyl-functionalized CCS polymers, as described above. Figure 3 shows the resulting structures from hydroxyl-functionalized G1 to G4 CCS polymers (SP5 8). In concert with the acetonide materials, star polymers with generations 1 to 3 (SP5 8) all formed honeycomb-like morphologies, whereas G4 (SP8) showed a markedly different morphology ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2008, 18,

4 Scheme 2. Functional group transformations of core crosslinked star polymers. Films cast from the hydroxyl-g1 star polymer (SP5, Fig. 3a and b) showed perfect monodisperse honeycomb films with pore sizes of 0.4 mm and multiple layers of pores. The hydroxyl- G2 star polymer (SP6, Fig. 3c and d) also showed a highly regular array of pores packed in a honeycomb morphology with a similar pore size (0.45 mm). Hydroxyl-G3 star polymers also displayed highly uniform honeycomb morphology with an increased pore size of 1.0 mm, and seemingly interconnected pores. However, a distinct difference is noted in this film; in this case, the open pore size is equal to the cell size, unlike the morphologies displayed from G1 and G2, where there is a larger cell size underneath the open pore. Hydroxyl-G4 showed a different, significantly disordered morphology with a larger cell underneath the skin of the porous film and multiple pore openings per cell, which are nonspherical, stretched shapes. It has been reported that amphiphilic polymers can act as stabilizers to facilitate the preparation of ordered honeycomb materials. [34] In that work, Shimomura and co-workers claim Figure 2. Scanning electron microscopy (SEM) images of honeycomb films cast from acetonide-functionalized dendron stars; a,b) G1-functionalized star polymer (SP1); c,d) G2-functionalized star polymer (SP2); e,f) G3-functionalized star polymer (SP3); g,h) G4-functionalized star polymer (SP4). Figure 3. SEM micrographs of honeycomb films cast from hydroxyl-functionalized dendron stars; a,b) G1-functionalized star polymer (SP5); c,d) G2-functionalized star polymer (SP6); e,f) G3-functionalized star polymer (SP7); g,h) G4-functionalized star polymer (SP8). Adv. Funct. Mater. 2008, 18, ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

5 that the amphiphilic-type polymers can reduce the surface tension between the water droplets and the volatile solvent, thus, further stabilizing the water droplets. The results presented in Figure 3 support this theory. By carefully varying the number of polar chain ends, the hydroxyl-g1 and G2 star polymers have the optimal amphiphilic character to stabilize the water droplets, thus, creating well-defined honeycomb materials over large length scales. Increasing the functionality further, the hydroxyl-g3 functionalized star polymer leads to a new effect. In a proposed mechanism for the BF technique, the precipitating polymer layer is pulled around the water droplet by the high surface tension of the water, creating a smaller pore opening at the surface of the film. This morphology can be seen in Figure 3a d (G1 G2). As observed in the G3 film (Fig. 3e and f), the smaller pore opening is not present; this could be caused by a lowering of the surface tension such that the water droplet does not pull the polymer layer around the droplet. Increasing the number of polar chain ends further, the G4 (SP8)-functionalized star polymer, with theoretically about 190 Figure 4. SEM micrographs of honeycomb films cast from perfluoroalkyl-functionalized dendron stars; a,b) G1-functionalized star polymer (SP9); c,d) G2-functionalized star polymer (SP10); e,f) G3-functionalized star polymer (SP11); g,h) G4-functionalized star polymer (SP12). Corresponding X-ray photoelectron spectroscopy (XPS) spectra of the films are given on the right-hand side. hydroxyl groups per star molecule, formed a different morphology (Fig. 3g and h). This could be the result of a further decrease in the surface tension of the water droplets. The mobility of this star polymer may also have an effect similar to the acetonide G4-functionalized star polymer Honeycomb Materials from Fluorine-Functionalized Star Polymers Porous films were also formed from perfluoroalkyl-functionalized CCS polymers, using the method described above. Figure 4 shows the resulting structures from G1 to G4- functionalized CCS polymers (SP9 12). On the right-hand side are the results from X-ray photoelectron spectroscopy (XPS) of the corresponding films; the strong silicon and sodium peaks arise from the glass substrate. The insets of the XPS data highlight the region where fluorine atoms can be detected. This series of star polymers, with nonpolar fluorinated end groups, displayed markedly different morphologies to the acetonideand hydroxyl-functionalized star films. Films cast from the perfluoroalkyl-g1 star polymer (SP9) showed a honeycomblike morphology with poor order and large pore size distribution (Fig. 4a and b). Figure 4b shows evidence of droplet coalescence which could give rise to the poor order of these films, and, from XPS data, fluorine atoms could not be detected. In contrast, the perfluoroalkyl-g2 star polymer (SP10) produced monodisperse, highly ordered honeycomb materials (Fig. 4c and d) with a pore size of 2.5 mm. Interestingly, a new type of morphology is observed, where pores with an oval shape are produced. In addition, there is no surface skin for the pores, which leads to very narrow wall thicknesses (380 nm) and again fluorine was still not detectable by XPS on the surface of this film. The perfluoroalkyl-g3 star polymer (SP11) showed a similar morphology to the perfluoroalkyl-g2 honeycomb structures; highly ordered, monodisperse honeycomb materials with a further increase in pore size to 3.0 mm were observed (Fig. 4e and f). The walls of this sample were even thinner than the generation 2 star polymer (220 nm) and fluorinewasdetectedbyxpsonthesurfaceof the G3 film (right-hand side of Fig. 4f). Films cast from the perfluoroalkyl-g4 (SP12) star polymer did not form honeycomb films (Fig. 4g and h) and the increased hydrophobic nature of the polymer was evident by severe dewetting on the hydrophilic glass cover slip. Further investigations utilizing hydrophobic and fluorinated substrates are currently underway ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2008, 18,

6 2.5. Comparison of Functional Groups In the previous sections it has been demonstrated that by changing the dendron size and the end-group functionality, vastly different morphologies have been produced. Some of the possible effects have already been described. Here, a comparison of the cross-sectional morphologies for the honeycomb films cast from G3-functionalized star polymers will be made. Figure 5 shows the morphology underneath the surface of these honeycomb materials. This figure highlights the different structures obtained by using the same star polymer but changing the functionality of the dendron end group at the periphery. Specifically, Figure 5 displays structures formed from G3-functionalized star polymers with acetonide functionality (Fig. 5a), hydroxyl functionality (Fig. 5b), and perfluoroalkyl functionality (Fig. 5c) and gives some insights into the different behavior of these materials. The acetonide-g3- functionalized star forms a honeycomb material in a similar fashion as reported previously for a poly(methyl methacrylate) star polymer, with seemingly independent and multilayered pores (2 4 layers) (Fig. 6). [21] When the end group is changed to hydrophilic hydroxyl groups, the morphology is slightly different, though the film still displays multiple layers (2 4 layers) but now with interconnected pores (Fig. 5b). The possible reason for this was alluded to earlier; with a high number of hydroxyl groups, the surface tension may be such that the film of polymer on top of the water droplet is not stable, thus, creating slightly larger pore sizes. The water droplets may coalesce in the final stages of the film formation, creating interconnected pores whilst maintaining some degree of order. Casting the same star polymer with a perfluoroalkyl end functional group showed a very different morphology. The resultant film had a monolayer of independent pores with very high order (Fig. 5c). The pores were also nonsymmetrical with the height of the pore being approximately twice the size of the pore opening. This high hydrophobicity may not enable the precipitating polymer to stably encapsulate the water droplet; instead the water droplet simply sinks into the solution, creating a monolayer of deep pores. As the formed porous structure is very unique it can potentially be used as a novel replica molding template, forming cylindrical surfaces features for electronic and optic devices. A summary of the different morphologies cast from G3-functionalized star polymers with varying end groups is schematically depicted in Figure 6. Figure 5. SEM micrographs taken at a 608 tilt of the honeycomb materials made from dendron-functionalized star polymers. a) Acetonide-functionalized G3 star polymer; b) hydroxyl-functionalized G3 star polymer; c) perfluoroalkyl-functionalized G3 star polymer. Insets show end-group structure Surface Properties of Honeycomb Films Cast from Star Polymers with Different End-Group Functionalities Water repellency is a growing area of research, [35] with the contact angle of a material being affected not only by its chemical structure (its inherent hydrophobicity) but also by the material s micro/macrostructure. [36] The porous materials prepared by the BF technique have potential applications as superhydrophobic/self-cleaning materials. Shimomura has shown a number of elegant examples of superhydrophobic honeycomb surfaces. [37] Here, we found that by changing the hydrophobicity of the star s end group, and the relative amount of functionality (changing generation), a change in the hydrophobicity of the resulting honeycomb surfaces can occur. It is also demonstrated that the porous structure of the honeycomb films also influence the hydrophobicities of the surface. All of the honeycomb surfaces described above (Figs. 2 4) were analyzed by water contact angle measurements (Fig. 7). In addition, all star polymer compositions were cast in dry conditions forming featureless (or smooth) films and their water contact angles analyzed. The featureless films (solid shapes in Fig. 7) were relatively constant with increasing generation. Only the perfluoroalkyl-functionalized films displayed a small increase in contact angle for the first three generations, until perfluoroalkyl-g4 star polymer, which slightly decreased, most likely due to a poor surface coverage of the film. The honeycomb films on the other hand showed more interesting behavior. As expected the increased surface roughness markedly increases the contact angles. The Adv. Funct. Mater. 2008, 18, ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

7 acetonide-functionalized honeycomb films showed no change for the first three generations, with a constant contact angle of around Acetonide-G4 honeycomb film decreased to just below 1008, which may be due to the closed nature of some of the pores. The hydroxyl-functionalized honeycomb films showed a regular decrease in contact angle with increasing generation, decreasing from 1088 (G1) to 958 (G4). This may simply be due to the increased number of polar hydroxyl groups present with the increasing generations, effectively enhancing the hydrophilic character of the films. The perfluoroalkylfunctionalized honeycomb showed a linear increase in the contact angle with increasing generation from G1 to G3 (1088 to 1228, respectively) and is attributed to an increasing amount of hydrophobic perfluoroalkyl chain, thus, increasing the hydrophobicity of the material. The perfluoroalkyl-g4 star polymer film showed a markedly lower contact angle (858); we believe this is due to the poor coverage of this film, causing the water to make some contact with the hydrophilic glass cover slip, lowering the contact angle. 3. Conclusions Figure 6. Schematic representation of morphology changes as a result of changing the end group of the G3-functionalized star polymers. Figure 7. Contact angle measurements of honeycomb materials made from dendron-functionalized stars. Open circles: honeycomb films cast from acetonide-functionalized star polymers (SP1 4); open diamonds: honeycomb films cast from hydroxyl-functionalized star polymers (SP5 8); open squares: honeycomb films cast from perfluoroalkyl-functionalized star polymers (SP9 12). Closed circles: flat films cast from acetonide-functionalized star polymers (SP1 4); closed diamonds: flat films cast from hydroxyl-functionalized star polymers (SP5 8); closed squares: flat films cast from perfluoroalkyl-functionalized star polymers (SP9 12). This work has demonstrated the effect of functional groups and macromolecular architecture on the BF technique, elucidating important insights into possible mechanistic features of this dynamic process. Functional-group modification of dendronized star polymers is shown to be a powerful technique that allows the synthesis of a range of stars with different functional groups and with varying amounts of these functional groups; this has been achieved through varying the dendron generation to be prepared. This controlled synthesis strategy enables a systematic study, showing the importance of structural control and the dramatic impact that chain-end chemistry can have on the final honeycomb structure. Increasing the polarity of the polymer precursor (number of hydroxyl groups per arm of star) shows an increase in order until, with the G3-functional star, the morphology changed into a thin-walled interconnected honeycomb film. Increasing the polarity further to G4 caused coalescence of water droplets and loss of order. Utilizing perfluoroalkyl-end-functionalized star polymers results in a novel porous morphology with extremely thin walls compared to the pore size and is only observed for a critical perfluoroalkyl content (G2 G3). These results demonstrate an important relationship between polymer functionality and ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2008, 18,

8 their behavior at the water interface (during breath-figure formation), which allows superior control over honeycomb films to be obtained and permits the purposeful design of desired morphologies. 4. Experimental Materials: All chemicals were purchased from Aldrich. Dowex- 50W-X2 was washed several times with methanol and then activated with 1 M hydrochloric acid in methanol. Pentadecafluorooctanoyl chloride (97%), tetrahydrofuran (THF, þ99.9%), and 4-(dimethylamino)pyridine (DMAP, þ99%) were used without further purification. Water was purified by a Millipore system (Milli-Q-Millipore). Preparation of acetonide-protected 2,2-bis(methoxy)propionic acid (bis-mpa) atom transfer radical polymerization initiators, acetonideprotected bis-mpa dendron-functionalized polystyrenes, and acetonide-protected bis-mpa dendron-functionalized star polymers (SP1 4) are described elsewhere [28]. Methods: Gel permeation chromatography multiangle laser light scattering (GPC-MALLS) was performed on a Shimadzu system with a Wyatt Dawn DSP multiangle laser light scattering detector (690 nm, 30 mw) and a Wyatt Optilab EOS interferometric refractometer (690 nm). THF was used as the eluent with three Phenomenex phenogel columns (500, 10 4, and 10 6 Å porosity, respectively; 5 mm bead size) operated at 1 ml min 1 with the column temperature set at 30 8C. Astra software (Wyatt Technology Corp.) was used to process the data using known dn/dc (specific refractive index increment) values to determine the molecular weight or an assumption of 100% mass recovery of the polymer where the dn/dc value was unknown. Scanning electron microscopy (SEM) was conducted with an XL 30 Philips Head SEM instrument. Samples were coated with gold using a Dynavac minisputter coater prior to imaging. Sessile-drop surface tension measurements were conducted on a DataPhysics OCA20 tensiometer, by casting a droplet (10 ml) of milliq grade water and measuring the contact angle measurements were repeated five times, and the average data was reported here. General Procedure for the Deprotection of Acetonide-Functionalized Star Polymers (SP5 8): Acetonide-protected star polymers (200 mg) were dissolved in THF/methanol in a 2:3 ratio (20 ml). The Dowex H þ resin (1 g) was added, and the reaction mixture was stirred (18 h) at 50 8C. The deprotection of the acetonide group was monitored by 13 C NMR spectroscopy, whereby the quantitative removal of the quaternary carbon was monitored by the disappearance of the signal at around 100 ppm. The Dowex H þ resin was then filtered off in a glass filter, and the methanol and THF were removed by rotary evaporation. Finally, the star polymers were redissolved in THF (5 ml) and precipitated in to methanol (100 ml). General Procedure for the Functionalization of Hydroxyl- Functional Star Polymers with a Perfluoroalkyl Chain (SP9 12): Pentadecafluorooctanoyl chloride (80 mg, 0.2 mmol) and DMAP (5 mg, 0.05 mmol) were dissolved in anhydrous THF (1 ml). The hydroxylfunctionalized PS star polymer (SP1) (80 mg) was dissolved in a mixture of dry pyridine (0.5 ml) and anhydrous THF (5 ml) and added dropwise to the reaction mixture. The reaction was stirred for an 18 h period. The solution was filtered and the solvents evaporated under reduced pressure (ca. 30 mm Hg; 1 mm Hg ¼ Pa). The polymer was redissolved in a minimum amount of THF, precipitated into methanol (50 ml), and collected by filtration to afford a white solid, which was analyzed by GPC-MALLS. Preparation of Honeycomb Films: Humid air was generated by the mixing of wet and dry air, with the humidity level being controlled through variation of the mixing ratio as described elsewhere [38]. 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